4 research outputs found
Assessment of valley cold pools and clouds in a very high-resolution numerical weather prediction model
The formation of cold air pools in valleys under stable conditions represents an important challenge for numerical weather prediction (NWP). The challenge is increased when the valleys that dominate cold pool formation are on scales unresolved by NWP models, which can lead to substantial local errors in temperature forecasts. In this study a 2-month simulation is presented using a nested model con- figuration with a finest horizontal grid spacing of 100 m. The simulation is compared with observations from the recent COLd air Pooling Experiment (COLPEX) project and the modelâs ability to represent cold pool formation, and the surface energy balance is assessed. The results reveal a bias in the model long-wave radiation that results from the assumptions made about the sub-grid variability in humidity in the cloud parametrization scheme. The cloud scheme assumes relative humidity thresholds below 100 % to diagnose partial cloudiness, an approach common to schemes used in many other models. The biases in radiation, and resulting biases in screen temperature and cold pool properties are shown to be sensitive to the choice of critical relative humidity, suggesting that this is a key area that should be improved for very high-resolution modeling
A caseâstudy of coldâair pool evolution in hilly terrain using field measurements from COLPEX
A caseâstudy investigation of coldâair pool (CAP) evolution in hilly terrain is conducted using field measurements made during IOP 16 of the COLdâair Pool EXperiment (COLPEX). COLPEX was designed to study coldâair pooling in smallâscale valleys typical of the UK (âŒ100â200âm deep, âŒ1âkm wide). The synoptic conditions during IOP 16 are typical of those required for CAPs to form during the night, with high pressure, clear skies and low ambient winds. Initially a CAP forms around sunset and grows uninterrupted for several hours. However, starting 4âhr after sunset, a number of interruptions to this steady cooling rate occur. Three episodes are highlighted from the observations and the cause of disruption attributed to (a) wave activity, in the form of gravity waves and/or KelvinâHelmholtz (KH) instability, (b) increases in the aboveâvalley winds resulting from the development of a nocturnal lowâlevel jet (NLLJ), (c) shearâinduced mixing resulting from instability of the NLLJ. A weakly stable residual layer provides the conditions for wave activity during Episode 1. This residual layer is eroded by a developing NLLJ from the top down during Episode 2. The sustained increase in winds at hillâtop levels â attributed to the NLLJ â continue to disrupt the CAP through Episode 3. Although cooling is interrupted, the CAP is never completely eroded during the night. Complete CAP breakâup occurs some 3.5âhr after local sunrise. This caseâstudy highlights a number of meteorological phenomena that can disrupt CAP evolution even in ideal CAP conditions. These processes are unlikely to be sufficiently represented by current operational weather forecast models and can be challenging even for highâresolution research models